The Skeleton of Giraffes

The giraffe skeleton consists of ~170 bones. The dry mass of the skeleton is 70 g.kg-1 body mass. The average chemical composition of their bones is 33% minerals (mainly calcium and phosphorus in a ratio of 2:1), 34% collagen, and 33% water. The skull contributes ~10%, the vertebrae ~25% and the limb bones ~65% to skeleton mass. The average density of all bones is 1.6 g cm-3, ranging from 0.8 g cm-3 (cervical vertebrae) to 2.0 g cm-3 (limb bones). Resistance to fracture by vertebrae depends on their cross-sectional area, and is greatest in cervical and the first few thoracic vertebrae. Resistance to fracture by limb bones depends on wall thickness (the difference between inner and outer diameter), which is uniquely thick. The growth of all limb bones except the humerus follows a geometric pattern (length and diameter increase at the same rate) which confers resistance to compression stress. The humerus follows an elastic pattern (diameter increases faster than length) a pattern that resists bending stress. Giraffes bones are exceptionally straight which further reduces bending stresses. The torque generated by the mass of the head and neck is resisted by the ligamentum nuchae which is exceptionally well-developed in giraffes, extends from the lumbar vertebrae to the occipital crest, can have a diameter of ~10 cm, and can support loads of ~1.8 tonnes before rupturing. As a giraffe grows muscle cross-sectional area (and contraction strength) declines and the duty factor reduces, both of which reduce the risk of fracture.